/Engine/Source/Runtime/InteractiveToolsFramework/Public/BaseGizmos/CombinedTransformGizmo.h
Note that this is an optional feature, as it would require tools to keep this transform up to date if they want the UI to use it, so tools could just leave it unset.
void SetDisplaySpaceTransform ( [TOptional](API\Runtime\Core\IO\TOptional)< FTransform > TransformIn )